Carbonate radical anion as an efficient reactive oxygen species: Its reaction with guanyl radical and formation of 8-oxoguanine

Mechanisms of reactions of four different tautomers (G(-H1) ^·, G(-H2a) ^·, G(-H2b) ^· and G(-H9) ^·) of guanyl radical with carbonate radical anion (CO ₃ ^.-) producing the mutagenic product 8-oxoguanine (8-oxoG) have been investigated theoretically using density functional theory and second order Moller-Plesset perturbation (MP2) theory. Geometries of reactant, intermediate and product complexes and those of transition states were optimized in gas phase. Solvent effect of aqueous media was treated by including eight specific water molecules in a hydration cell during optimization and single point energy calculations employing the polarizable continuum model. It is found that the specific water molecules catalyze proton transfer involving the different sites of the guanyl radical during the reaction. Possible roles of aeration, stirring and photoirradiation of reaction media which are employed to facilitate the reaction in experimental studies are explained. The carbonate radical anion is found to be a very effective reactive oxygen species.